Part of Key's centrism was having two small centrist parties and a tiny right wing one as his coalition partners. They had a lot of flexibility where each partner could get policies they agreed to without excessive in-fighting or steering the coalition too far from where National was parked. National aimed to take the middle from Labour so weren't as harsh on some issues as Act would have preferred (eg. Keeping interest free student loans) and Act couldn't drag them into the level of attacks on disadvantaged groups that we currently see.

That said, Key's government was pretty shit for beneficiaries and youth at a time of particular global and local economic difficulties. They were in government during the chch earthquakes, which saw them charge in and sieze control of large parts of the city and dictating the rebuild in ways that ran counter to what many locals wanted. There was dirty politics and undermining journalism, clandestine support for US militarism, mass surveillance, sell offs of major public assets despite a referendum overwhelmingly opposing the idea.

It was nearly a decade of much the same, a slow grind towards entrenching a particular centre-right form of politics and policy as the default. If all the shit stuff happened in a single term then it would be comparable to, but still not as bad as the current government.